Illumination sources are required for many applications, including displays for a wide variety of computers and consumer devices such as TVs. Illumination sources based on fluorescent lights are particularly attractive because of their high light output per watt-hour of power consumed. However, such sources require high driving voltages, and this makes them less attractive for battery-operated devices.
As a result, there has been considerable interest in utilizing light sources based on LEDs in such applications. LEDs have better electrical efficiency than incandescent light sources and longer lifetimes than both incandescent and fluorescent light sources. In addition, the driving voltages needed are compatible with the battery power available on most portable devices. Finally, continued advancements in the efficiencies of LEDs hold the promise of providing a light source with significantly higher efficiencies than fluorescent light sources. Unfortunately, LEDs suffer from three drawbacks with respect to their spectral output.
One drawback is that LED emission in the visible part of the electromagnetic spectrum is relatively narrow, with typical FWHM of a few tens of nanometers. Therefore, an LED light source for generating an arbitrary color of light typically requires a plurality of LEDs. The relative intensities of the LEDs are adjusted by adjusting the average drive current through each of the LEDs. Feedback loops that depend on the measurement of the light output from each of the LEDs, or groups of LEDs, are typically utilized to adjust the average drive currents through the LEDs.
If each LEDs light output as a function of drive current remained constant over the life of the LED, the relative intensities of the LEDs could be set once at the factory. However, LEDs suffer from aging problems. As an LED ages, the average drive current through the LED must be increased to compensate for the aging of the LED. Since the aging effects are different for different color LEDs, the perceived color of the illumination source, and hence, the color of any associated display, will change over time, unless the drive currents are altered to compensate for the aging. In one class of light sources, the intensity of light in each of the color bands is measured by a corresponding photodetector. The drive conditions are then adjusted to maintain the output of the photodetectors at a set of predetermined values corresponding to the desired perceived color for the light source. This approach requires a design in which the photodetectors sample the light that is generated by the LEDs, and a feedback loop is provided between the photodetectors and the LEDs to compensate for the aging effects discussed above.
The light sensor used in the feedback loop typically includes one or more photodiodes. In some arrangements, one photodiode is used to measure each group of LEDs that emit light of a particular color. The output of the photodiode is used in a servo loop to adjust the average current through that group of LEDs. Unfortunately, the photodetectors in current use are sensitive not only to visible wavelengths, but also to infrared wavelengths, and hence, the output of the photodetector is not purely a function of the light of interest when infrared light is also present in the light received by the photodetector. The source of the infrared light can be the LEDs or ambient light sources that emit light that is also captured by the photodetectors. Hence, a feedback loop that operates on the signal from one of the photodetectors will not function properly unless the effects of the infrared can be eliminated. Since the amount of infrared reaching the photodetectors can vary over time, either the infrared reaching the photodetector must be measured and used to correct the feedback loop or some form of infrared filter must be incorporated in the feedback loop to block infrared from reaching the photodiodes in the photodetector.
In many applications, the infrared filter is incorporated into the package containing the photodetector. In the prior art, the filter is secured with adhesive to the top surface of the material encapsulating the die or dies of the photodetector package. Unfortunately, the thermal expansion coefficient of the filter material is typically very different from that of the underlying encapsulant. This mismatch leads to device failure when the package is subjected to large temperature cycles. For example, when the photodetector is mounted on a printed circuit board, or the like, using a solder reflow process.